Parallelograms which are on the same base and between the same parallels are equal to one another. Let $ABCD$ and $EBCF$ be parallelograms on the same base $BC$, and between the same parallels $AF$ and $BC$. I say that $ABCD$ is equal to parallelogram $EBCF$. <prf> Thus, the whole parallelogram $ABCD$ is equal to the whole parallelogram $EBCF$.Thus, parallelograms which are on the same base and between the same parallels are equal to one another.